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BQ25708: BQ25708 does not wakeup batteries from deep sleep

Martin Mensch
Prodigy 80 points
Part Number: BQ25708

Hi,

we designed a charger around BQ257078 and in general it works fine. Now we connected a battery in deep sleep, that has its SM-Bus deactivated. But the charger around BQ25708 does not put any voltage on the battery Vbat so it is not woken up and not discovered by the system. We have register MinSystemVoltage (0x3E) in default and see 12,288V (4S) as Vsys. This sentence from page 4 of the data sheet makes me think it should work like that: "BATFET is in linear mode to regulate VSYS at minimum system voltage when battery is depleted. "

What are we doing wrong? Any configuration issues?

Thanks for any help

Martin

  • 0
    TI__Expert 4780 points

    Hi Martin,

    Just to clarify, is this being done with Adapter connected, or are you trying to attach a battery without adapter? What is the battery voltage?

    Thanks,
    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 80 points

    Hi Khalid,

    yes the charger is supplied via the adapter side with 20 V. The battery voltage is 0V due to the battery being in deep sleep. The battery is not responding to requests on the SM-bus by the system host.

    I would expect that the battery should be charged with a small current to take it out of deep sleep and then when it requests a larger current by itself provide this current.

    What is the normal state of the charger being supplied from the adapter side? Is there a voltage on the bat terminal behind the mosfet ( the far right labeled BATT (1S-4S) on the application diagram) ?

    Thanks for any help

    Martin

  • 0 in reply to Martin Mensch
    TI__Expert 4780 points

    Hi Martin,

    I would expect a deeply depleted battery to be in the range of ~2-3V per cell. 

    In general the device senses the battery voltage using the SRP/SRN pins, and charging behavior depends on this. If the Battery Voltage is below VBAT_LOWV the device will charge with a trickle current (384mA). The system should still wake up with an adapter plugged in.

    You mentioned that "The battery is not responding to requests on the SM-bus by the system host." Do you mean that the BQ25708 is not responding to SMB at all?

    Khalid

  • 0 in reply to Khalid Bairuti
    TI__Expert 4780 points

    Hello Martin,

    Is there any update on this one?

    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 80 points

    Hello Khalid, hello all,

    after quite a number of experiments I found out these points:

    • Bit CHRG_INHIBIT in register ChargeOption0 (12h) will when set disable supplying a battery in deep sleep, so also the battery's SM-bus will not work and thus the battery can not be detected via SM-bus. This bit is kind of a main switch. Further points assume this bit is cleared.
    • Bit AUTO_WAKEUP_ENABLE in register ChargeOption1 (30h) is set by default after reset / power up. In this state the charger will supply a battery in deep sleep and thus make its SM-bus work. It will then charge it with a small current.
    • Bit AUTO_WAKEUP_ENABLE will be cleared once a somewhat chargered battery was seen (I don't have any clear information when precisely). This means that when not set again by host all further connections of batteries in deep sleep will fail
    • Bit AUTO_WAKEUP_ENABLE can't be set immediately after a battery was disconnected. I experienced up to 1 sec until setting the bit really took effect.

    I hope this helps someone else with similiar trouble.

    Martin

  • 0 in reply to Martin Mensch
    TI__Expert 4780 points

    Hi Martin,

    Thanks for posting the update with details.

    Khalid